In electrical installations in which a direct voltage generated by photovoltaic elements is converted into alternating voltage by means of transformerless inverters, capacitive leakage currents may occur that depend on the circuit and the modulation method selected, since the potential of the solar generator can fluctuate with respect to ground potential. This can influence a residual-current-operated protective device of the inverter negatively, for example.
One reason for the occurrence of leakage currents is an alternating-voltage component which, due to the circuit topology and the modulation of the inverter, is superimposed on the direct voltage generated by the photovoltaic elements. The leakage currents drain off to ground potential (PE) via so-called leakage capacitances (external ones of the photovoltaic element and those inside the inverter).
Although it is attempted to reduce this effect by optimizing the inverter topology, the effect of capacitive leakage currents can, however, occur in spite of such an optimization in certain operating states even in the case of transformerless inverters of more recent construction, especially in the case of inverters that are not operated in the conventional sine-delta-modulation.
Such a transformerless inverter of more recent construction, especially optimized and advantageous with regard to its efficiency, in so-called “three-point topology” is disclosed in EP 2 107 672 A2. It shows a three-phase transformerless inverter with a link, the inputs on the direct-voltage side of which are connected to one another by two series-connected capacitances, the two capacitances defining a center voltage point that is not connected to the neutral conductor of the alternating-voltage grid.
In such a three-point topology, the voltage at the input UDC to PE is, in case of normal pulse width modulation (PWM) that is generated in accordance with the common principle of sine-delta modulation, a direct voltage so that in this case, in which a link voltage at the input of the inverter bridge must be at least as large as twice the peak voltage of the line voltage signal so that the modulation degree of the PWM is less than or equal to 1, initially no leakage currents occur. If the link voltage is reduced to a value below twice the peak voltage of the line voltage signal, this results in overmodulation for the PWM, i.e. a modulation degree of greater than 1, which leads to distortion for the current at the inverter output.
To achieve as good an efficiency as possible, it is advantageous to keep the link voltage as low as possible. For this purpose, methods are known, especially for three-phase inverters, in which, for example, in the case of conventional sine-delta modulation, a time-variant offset, for example a delta signal with three times the line frequency or a sine signal with three times the line frequency is added so that, in spite of the occurrence of link voltage values below twice the peak voltage of the line voltage signal in the PWM signal itself, no overmodulation occurs, and thus also no current distortion of the inverter output signal fed into a grid occurs. Such modulation methods include, for example, also the methods known by the terms “space vector modulation” or “sine-delta modulation with third harmonic”.
In the case of modified sine-delta modulation methods, especially in the case of the aforementioned methods of sine-delta modulation with time-variant offset, an alternating voltage component with three times the line frequency occurs, for example, but not only, in the aforementioned transformerless inverter from EP 2 107 672 A2, due to its topology, between the voltage center point at the input and PE, which causes a capacitive alternating current via the leakage capacitances (leakage current).
Inverters usually have a residual-current-operated protective device, which detects the differential currents on the alternating-current side and, in the case of a fault, e.g., in order to avoid danger to persons or damage to the inverter itself, switches the inverter off. However, the measured differential current contains not only the fault current actually to be detected but additionally in a vectorial sum also a leakage current, which may be present, so that leakage currents that are too large can lead to an erroneous triggering of the residual-current-operated protective device and, thus, to an unwanted switching-off of the inverter. Refraining from the use of modulation methods in which leakage currents can occur due to topology will again worsen the efficiency and, therefore, does not represent a suitable solution to this problem.